1. Field of Invention
The present invention relates generally to the field of transmitters in mixed-signal communication channels of the kind used in serial transmissions.
2. Description of Related Art
Signal processing in digital communications is concerned with optimizing performance given the constraints of power, noise, bandwidth limitations, area, memory, sampling limitations and numerous other requirements depending on the architecture employed.
Data communication is moving away from parallel busses and towards serial transmission and serial protocols. System designers are under pressure to reliably send data over low bandwidth channels at rates for which they may not have been optimally designed for, and to pack more communication channels into smaller and smaller spaces. This results in increasingly poorer receive signal levels and higher levels of crosstalk, among other problems. Those who design transmitters for these systems need to find ways of improving the quality of the signals seen by the receiver.
A typical system is shown in FIG. 1. A transmitter 10 sends serial data, typically digital signals in the form of pulses, out through the channel 20, having a channel response transfer function H(s), and is picked up by a receiver 30. The output of the transmitter is a wide bandwidth digital signal. The channel 20 can be a mixed-signal channel utilizing both analog and digital signals. Channel 20, which could be a backplane, a PCB trace, a cable, an optical link, an internet or interchip connection, or any other communications channel, generally has been found to act as a lowpass function filter which filters out the high frequency energy from the transmit signal, as generally the channel is limited in bandwidth. By the time the transmitted signal from the transmitter passes through the communications channel 20 and is received by the receiver, the transmitted signal could be significantly degraded due to this loss of high frequency energy. This losses causes short (1 to 2 bit time) pulses to be narrowed in width (period) and reduced in height (amplitude).
One solution to this problem is to boost the high frequency components of a signal at the transmitter to compensate for the high frequency loss in the channel. This is frequently called transmit emphasis. But existing systems that employ transmit emphasis suffer from numerous disadvantages, as explained further herein.
Concomitantly, many receiver designs employ some sort of equalization to boost high frequency signals that are attenuated as the signal passes through the channel. This might be realized using a linear high pass filter, that boosts the high frequency signal was well any associated noise, and thus the signal-to-noise ratio (SNR) remains poor.
Another technique for handling attenuation of high frequency signals by communication channels is to employ decision feedback equalization/equalizers (DFE) at the receiver end, to help remove noise and distortion of digital signals, such as intersymbol interference (ISI) caused by attenuation of high frequencies. Some of the problems associated with digital signal processing as addressed by DFE are outlined in U.S. Pat. No. 6,437,932 to Prater et al., commonly assigned to the present assignee, and incorporated herein in its entirety.
Factoring all of the above, it can be seen that what is needed is a superior method and apparatus for reducing distortion in communication channels when transmitting digital data that contains high frequency signals in a mixed-signal communications channel.
To better understand the advantages of the present invention, FIGS. 2-3 disclose existing solutions, that the present invention improves upon.
Turning now to FIGS. 2A-2D, there are shown a idealized waveform diagrams for a typical system that employs transmit emphasis of a transmitted signal. The unemphasized transmit waveform sent by a transmitter, such as transmitter 10 in FIG. 1, is shown as waveform (A), FIG. 2A, labeled “Unemphasized Transmit Waveform”. The received waveform received by a receiver, such as receiver 30 in FIG. 1, after channel distortion caused by a channel, such as channel 20 having transfer function H(s) in FIG. 1, is shown as waveform (B), labeled “Received Waveform”, FIG. 2B. Waveform B has distortion, as can be seen by viewing bits 1, 9 and 13, inter alia, which have a height (amplitude component) and width (period component) less than ideal.
The idea behind transmit emphasis is for the high frequency components of the transmitted signal to be boosted at the transmitter side of the system, to compensate for the high frequency loss in the channel, so that the received signal will be improved. Thus the first bit of a digital pulse train, after each transition, is sent with a larger amplitude than subsequent bits, increasing the high frequency energy content relative to the low frequency energy content, since any transition or edge in a pulse train will have higher frequency components. This is shown graphically in FIG. 2C, as waveform (C), labeled “Emphasized Transmit Waveform”. Thus, at pulse transition 3 to 4 (from left of waveform), where there is no transition from LOW to HIGH in the Unemphasized Transmit Waveform, (A) in FIG. 1, for the Emphasized Transmit Waveform, the fourth pulse bit (bit 3) has a slightly different height (amplitude) than the third pulse bit; likewise, at pulse 9 (bit 8); pulse 13 (bit 12); pulse 16 (bit 15), the amplitudes are slightly different at the transition from their neighbors, thus having different emphasis, in order to increase the high frequency energy content relative to the low frequency energy content.
The waveform received after transmission of the Emphasized Transmit Waveform (C) through the channel is shown in FIG. 2D as waveform (D), labeled “Received Waveform”, which contains less distortion than received waveform (B), such as seen by comparing the two waveforms bits 1, 9 and 13.
The circuit to implement the Emphasized Transmit Waveform is shown in FIG. 3. A 2-tap FIR (Finite Impulse Response) filter is shown in the transmitter. The data to be transmitted, TXDATA, is retimed on clock line TXCLK and presented to the Main Transmit op-am driver 305 on timing signal line DOUT. The data to be transmitted is also delayed by the Delay flip-flop 309 and inverted, retimed and presented to the Emphasis driver op-amp 310 on timing signal line DOUTD for presentation to summer 312, with the aid of two Retiming Flip-Flops 307. As shown, the Emphasis driver 310 is has a programmable drive strength to control the amount of de-emphasis to increase the high frequency to low frequency energy content in the transition bits. The two driver outputs are combined at summer 312, with suitable inversion as necessary, as inversion generates de-emphases, to form TXOUT, the emphasized transmit signal output to the channel.
Notwithstanding the above, existing systems of transmit emphasis have several disadvantages in practice. In a transmit emphasis system such as shown in FIG. 2, ideally a system designer would set the transmit amplitude to maximize noise immunity, and then would use emphasis to increase the amplitude of the transition bits to improve the “eye” (a test pattern that measures the health of a communications channel). In reality, however, as semiconductor technology progresses, power supply voltages shrink. This makes it difficult to transmit large amplitude signals, so emphasis usually means transmitting maximum amplitude after the transitions and less than maximum amplitude on subsequent bits. The result is that in practice increased emphasis levels result in decreased available transmit amplitude.
Secondly, increasing the high frequency energy transmitted increasing the amount of crosstalk seen by receivers that may be nearby. This can turn a usable system into an unusable system.
Thirdly, increased high frequency energy in the transmit signal can exaggerate the effect of resonance in the channel and cause increased radiated EMI. Radiating EMI causes EMI compliance difficulties. Resonance can be due to reflections from connectors or stubs.
Fourthly, emphasis such as shown in the existing system of FIG. 3 requires at least one extra driver stage connected to TXOUT. This adds capacitance to the output, which reduces the bandwidth of the driver and increases reflections in the channel.
Lastly, channels that employ optics have additional amplitude limiting stages in the electro-optical conversion process. Amplitude modulation does not compensate past the electrical channel leading to the optics.
The present invention attempts to overcome the above-mentioned disadvantages and ameliorate the system of amplitude boosting high frequencies signals at the transmit end of a system employing a transmitter, communication channel and receiver.